The present invention relates to a multiplex transmission apparatus and a multiplex transmission method used when a controller performs data transmission/reception via a communication unit having a transmission buffer through a multiplex transmission path.
In recent years, in the field of automobiles, since the number of electrical components such as switches, sensors, actuators, and the like is increased according to the advance in "car electronics", an enlarged, complicated wiring harness structure for connecting these electrical components poses a serious problem. In order to solve this problem, as disclosed in Japanese Patent Laid-Open No. 63-142933, a multiplex transmission system which shares a single transmission path by a plurality of electrical components is receiving a lot of attention. When the common transmission path is time-divisionally used, data communications among a plurality of nodes connected to the transmission path is realized.
For example, a control node (4WS node) for performing four-wheel steering control comprises a CPU serving as a controller connected to sensors and actuators, and a multiplex LSI serving as a communication unit for controlling multiplex transmission. In order to realize the four-wheel steering control, the CPU controls the operation of its own actuators according to data from its own sensors, and issues a transmission request to the multiplex LSI, so that data from the sensors can be utilized by another node, e.g., a control node (TRC node) for performing traction control.
The multiplex LSI comprises a transmission buffer, and data corresponding to the transmission request is written in the transmission buffer. Furthermore, the multiplex LSI adds information indicating a transmission source and a transmission destination to the data in the transmission buffer, and transmits the data onto a multiplex transmission path in a predetermined format. In this case, when a plurality of nodes simultaneously execute transmission operations, signal collision occurs on the multiplex transmission path. In order to prevent this collision, a priority order is given to the nodes. That is, a node having a low priority must execute transmission after the transmission of a node having a high priority is completed.
The multiplex LSI of the 4WS node also comprises a reception buffer. That is, required data is extracted from a signal on the multiplex transmission path, which signal is transmitted from another node to the 4WS node, and the extracted data is written in the reception buffer as reception data. The CPU of the 4WS node reads the data in the reception buffer, and reflects the read data in the four-wheel steering control.
On the other hand, the TRC node also comprises a CPU and a multiplex LSI, which are the same as those in the 4WS node. That is, the TRC node reflects data transmitted from other nodes in its own traction control, and has a function of transmitting data to other nodes. Conventionally, cooperation control between, e.g., the 4WS and TRC nodes is realized, as described above.
However, according to the conventional multiplex transmission method, the CPU of, e.g., the 4WS node has no means for checking if data written in the transmission buffer is actually transmitted onto the multiplex transmission path. For this reason, when the CPU of the 4WS node reads reception data from the TRC node while the 4WS node is set in a transmission standby state, the cooperation control with the TRC node is interfered with.
More specifically, although data written in the transmission buffer and addressed to the TRC node is not transmitted yet in practice, the CPU of the 4WS node erroneously determines that the read reception data is a response to its own transmission data, and reflects the read data in the four-wheel steering control. On the other hand, since the CPU of the TRC node has not actually received transmission data from the 4WS node, it has not transmitted response data to the transmission data of the 4WS node yet. That is, control inconsistency occurs between the 4WS and TRC nodes, and preferable cooperation control between these nodes cannot be realized.